It is known that many fluid systems require precise measurement of the state of the system, and in particular various properties and/or parameters of the fluids flowing through them. In some of these systems measurements of single parameters are important. In other cases, the change or difference of parameters is important. In both cases the accuracy required for each particular fluid system can vary on the basis of the particular fluid or fluids involved, and/or on the basis of the aim of the system.
An example of a fluid system having special requisites which can be considerably influenced by the accuracy of the parametric measurements, in particular comprising the determinations of the pressure, is a blood flow system that is external of the body, also known as an extracorporeal blood system.
An extracorporeal blood system normally includes a device for blood treatment flowing internally thereof. There are various types of these devices. Filtration devices having semipermeable membranes are commonly used in extracorporeal blood systems such as those used for dialysis or for therapeutic plasmapheresis (TPE). The primary aim of a semipermeable membrane is normally to provide removal or separation of determined elements or components from blood. Urea and other waste products are removed from the blood during dialysis, and the blood plasma is separated from the red corpuscles during TPE. The blood or the red corpuscles processed are then returned to the patient.
In more detail, in an extracorporeal blood system using a semipermeable membrane device, the process is the following. The blood is removed from the patient, passed along a side of a semipermeable membrane and in contact therewith. Undesired portions of the blood (urea in the case of dialysis, plasma in the case of TPE) diffuse or are filtered through the pores of the semipermeable membrane. The blood remaining on the blood side of the semipermeable membrane is then returned to the patient with a smaller quantity of the undesired substances.
As mentioned herein above, the prior art describes monitoring the state of the blood lines of medical machines for extracorporeal blood treatment, for example for detecting the presence of any eventual stenoses, i.e. narrowings in the lines in which the blood runs, either partial or total, or other occlusions. The stenoses can be due to various causes, from a progressive coagulation of the blood to a narrowing due to accidental clamping of the line, a blockage in a blood treatment device arranged in the bloodline, or other causes. For example, document U.S. Pat. No. 6,623,443 describes a method for detecting stenoses in a blood access or in a line for extracorporeal blood treatment which comprises monitoring the amplitude of an oscillating pressure signal in the circuit itself and detecting the presence of eventual stenoses on the basis of a monitoring of the variations in the amplitude. This method can be performed with a control device that is not very complex, but enables detection of the presence of stenoses only after the stenoses have caused a significant variation in a pressure measured in the circuit.
This method therefore does not entirely prevent the risk of damage to some components of the circuit, or deterioration of the blood (hemolysis) in the case of stenosis, as the reaction time of the control device can in some cases be not sufficiently rapid, also because there can sometimes be sufficient pressure variations for brief periods of time to damage the blood or some components. Document US2002/0174721 describes another method for detecting stenoses in lines for extracorporeal blood treatment. The method comprises measuring an oscillating pressure signal, for example due to the thrust of the blood by a peristaltic pump, and to perform a frequency analysis of the oscillating pressure signal in order to detect the presence of a stenosis in a case of attenuation of the components of greater frequency of the signal. The method enables a more prompt detection of the presence of stenosis with respect to the previously-cited method, before the pressure variations due to the presence of the stenosis can take on relevant values that might be potentially dangerous for the blood line and for the blood. The method however requires a sophisticated and expensive control device, which is able to perform complex calculation, such as frequency analysis of a signal in real time.
Both the above-described methods further require the presence of a high number of pressure sensors in the extracorporeal blood treatment circuit, in order to detect the changes in pressures in the various parts of the circuit, with a consequent increase in the complexity and costs of the system. Further, these methods essentially allow only verification of stenoses in the blood line, while not enabling detection of further significant parameters relating to the functioning state of the extracorporeal blood circuit.